Generic Map/Reduce List Extensions in C#

Question

I am writing a few extensions to mimic the map and reduce functions in Lisp.

public delegate R ReduceFunction<T,R>(T t, R previous);
public delegate void TransformFunction<T>(T t, params object[] args);

public static R Reduce<T,R>(this List<T> list, ReduceFunction<T,R> r, R initial)
{
     var aggregate = initial;
     foreach(var t in list)
         aggregate = r(t,aggregate);

     return aggregate;
}
public static void Transform<T>(this List<T> list, TransformFunction<T> f, params object [] args)
{
    foreach(var t in list)
         f(t,args);
}

The transform function will cut down on cruft like:

foreach(var t in list)
    if(conditions && moreconditions)
        //do work etc

Does this make sense? Could it be better?

Answer 1

According to this link Functional Programming in C# 3.0: How Map/Reduce/Filter can Rock your World the following are the equivalent in C# under the System.Linq namespace:

• map --> Enumerable.Select
• reduce --> Enumerable.Aggregate
• filter --> Enumerable.Where

Answer 2

These look very similar to extensions in Linq already:

//takes a function that matches the Func<T,R> delegate
listInstance.Aggregate( 
    startingValue, 
    (x, y) => /* aggregate two subsequent values */ );

//takes a function that matches the Action<T> delegate
listInstance.ForEach( 
    x => /* do something with x */);

Why is the 2nd example called Transform? Do you intend to change the values in the list somehow? If that's the case you may be better off using ConvertAll<T> or Select<T>.

Answer 3

I would use the built in Func delegates instead. This same code would work on any IEnumerable. Your code would turn into:

public static R Reduce<T,R>(this IEnumerable<T> list, Func<T,R> r, R initial)
{
     var aggregate = initial;
     foreach(var t in list)
         aggregate = r(t,aggregate);

     return aggregate;
}
public static void Transform<T>(this IEnumerable<T> list, Func<T> f)
{
    foreach(var t in list)
             f(t);
}